Servo guided stages are known. See, for example, U.S. Pat. No. 5,140,242 to Doran et al, entitled "Servo Guided Stage Systems" which is incorporated herein by reference, and includes a drive stage as provided in FIG. 1.
FIG. 1 shows the electrical schematic diagram of the control system for a stage 20. Three identical velocity servos shown in FIG. 2 are used to move the three capstan drive units 22, 24 and 26. When the servo control electronics 28 receives a new destination from a host computer 30 on lines 32, a series of velocity values are sent to the velocity servos 22, 24 and 26 to cause the stage 20 to move to the desired new destination. The closed loop position servo loop gain vs frequency and the maximum values of stage velocity, acceleration and rate of change of acceleration are controlled by stored parameters and software in the servo control electronics 28.
An X-Y joystick 39 also provides input to the control electronics 28 for manual control of the position of the x-y-theta stage 20.
The Laser Position Transducer and Servo Control electronics 28 receives the X position signals from the output of the X axis receiver 34 through cable 36. Control electronics 28 also receives the output of Y-axis receiver 38 through cable 40. Electronics 28 also receives the output of theta receiver 42 through cable 44. Interferometer 46 employs a pair of light beams directed to target mirror 48. For the Theta and Y axis measurements by receivers 38 and 42, a pair of interferometers are housed in an integrated structure 50. Laser beams 52 and 54 pass to the Theta receiver 42 and Y-axis receiver 38, respectively, from the interferometers in structure 50. The interferometers operate with the target mirror 56 where the pair of light beams for each are shown as single beams 58 and 60, respectively, for convenience of illustration.
The electronics 28 have X-error output 62 to the positive input of summing circuit 64 which provides an output 70 to X-drive amplifier 66 which energizes motor 22. The tachometer feedback 68 connects to the negative input of summing circuit 64 to provide negative feedback.
The electronics 28 also have a Y-error output 22 to the positive input of summing circuit 74 and summing circuit 76. Summing circuit 74 provides an output 78 to Y+Theta drive amplifier 80 which energizes motor 26 through drive line 82. The tachometer feedback on line 84 is connected to the negative input of summing circuit 74 to provide negative feedback. The electronics 28 further have a Theta-error output 86 to the negative input of summing circuit 76 and the positive input of summing circuit 74.
Summing circuit 76 provides an output 87 to Y-theta drive amplifier 88 which energizes motor 24 through line 90. The tachometer feedback 92 connects to the negative input of summing circuit 76 to provide negative feedback.
The stage is moved by three drive bars 94, 98, and 96 which are driven by capstans 22, 24, and 26 which are rotated by motors with integral tachometers. The motors are driven by power amplifiers 80, 88 and 66. The rotational speed of each capstan and, ultimately, the linear velocity of the drive bar and stage is controlled by three velocity servos. Each velocity servo loop is composed of a driver, motor, tachometer and a summing junction. The driver supplies current to the motor which accelerates the motor until rotational speed measured by the tachometer equals the error signals (X ERROR, Y ERROR, or Theta ERROR) coming into the summing junctions.
When the servo control electronics 28 receives a new destination from the host computer 30 on lines 32, a series of velocity values are sent to the velocity servos driving capstans 22, 24 and 26 which cause the stage 20 to move to the desired new destination.
When pure X motion is desired, the host computer loads a new X destination. A series of X velocity commands are given to move the X motor 22 and drive bar 94 until the stage position error is driven to zero at the new X location. While the stage is moving in the X axis, the Y position is interferometrically monitored and in response to disturbances in its Y position, the Y position servo is driving the Y motors 24 and 26 to actively keep the Y position error near zero. Also during the X move, Theta disturbances will occur. So, the Theta is also interferometrically monitored and in response to detected variations in Theta, the Theta servo actively drives the Y motors 24 and 26 differentially to keep the YAW near zero during the move and to hold it near zero after the move.
The Theta servo maintains the Theta (YAW angle) of the stage near zero by electronically superimposing small velocity corrections 86 to the Y velocity commands 72 in summing junctions 74 and 76. The dedicated closed loop Theta servo is constantly compensating for tiny gain differences in amplifiers 80 and 88 and motors 24 and 26, inertia differences in the stage and drive bars 96 and 98 and dynamic friction differences in the bearing pads. The Theta servo also removes the Theta disturbances caused by the X drive bar 94. Since the task of maintaining a small value of Theta is accomplished by a dedicated Theta servo, the Y position servo hardware and software can be identical to the X position servo.
If a Theta value other than zero is desired, the host computer loads the Theta value into the electronics 28 and the Y drive bars 96 and 98 are pushed and pulled by motors 24 and 26 as required to achieve the new Theta value.
Pure Y motion can be achieved by loading a new Y destination and maintaining the same X position. When the stage is driven with Pure X motion or Pure Y motion, the direction in which the stage is driven is called the "on-axis" direction. The othogonal direction to the on-axis direction is called the "off-axis" direction. Typically, the stage is stepped incrementally in either the X or Y direction. When the stage is stepped, the servo develops an error signal (not shown) internal to the electronics module 28, that is the difference between the actual position of the stage 22 and the preferred stage (X, Y) location. This error is known as a following error.
In FIG. 2, the stage 20 is shown centrally positioned relative to its base 100. For illustration, a workpiece 102 is held on the stage. FIG. 3 is a plot of the following error for a 2 mm Pure X step for the stage 20 positioned in FIG. 2. As shown by the "Y" marked trace, the off-axis following error for a 2 mm step in the position of FIG. 2 is inconsequential. Using the normal requirement that to be considered in place, the stage must be within 5 .mu.m of its objective endpoint coordinates, the move is complete after 100 ms. However, for this prior art stepper, it was found that the average stepping time across the entire stage field is appreciably longer than 100 ms. Furthermore, it was found that the off-axis following error in prior art steppers is not uniform in virtually every stepper stage location. This non-uniformity in following error results in longer stepping an settling times that degrade stepper throughput.
For example, with the stage in the off-center location of FIG. 4, the following error for a 2 mm on-axis step (in the x direction) is shown in FIG. 5. The on-axis following error in FIGS. 3 and 5 is nearly identical for the first 70 msecs. Conversely, the off-axis following error is dramatically higher in FIG. 5. The off-axis following error in this off-center position lags the on-axis following error and, eventually, as the off-axis following error dissipates, it induces a backlash effect in the on-axis position that results in an additional on-axis following error. Furthermore, in this off-center position, neither the on-axis, nor the off-axis following error settles to within 5 .mu.m until about 135 ms have passed. Thus, in the position of FIG. 3, off-axis following error is almost equal to on-axis and move time is 35% longer than for stage in the position of FIG. 1.
In a step and repeat lithography system, move time is overhead time. Typically, exposing a single layer requires 6K steps. Consequently, the wafer handling time for each wafer exposure could average 6K.times.0.05 secs=5 minutes, thereby significantly impacting wafer throughput.
This off-axis following error in prior art stepper stages occurred because the positioning servo loop does not have sufficient bandwidth to respond quickly enough to the unanticipated disturbance from on-axis motion.